The invention relates to improvements in methods of and in apparatus for applying viscous material to substrates, e.g., for applying an adhesive or a sealing compound to a workpiece. More particularly, the invention relates to improvements in methods of and in apparatus for applying strips of viscous material with assistance from compressed air or from another compressed gaseous fluid.
Viscous materials of the character under consideration here are those exhibiting a rather high viscosity, preferably higher than about 100 cSt. Such materials are not likely to be atomized by compressed gas but will retain the form of a continuous strand, layer or strip, or at least the form of sections of a layer, strand or strip. An optimum viscosity can be arrived at by varying the temperature and/or by adding effective amounts of a suitable solvent.
U.S. Pat. No. 4,995,333 granted Feb. 26, 1991 to Keller et al. discloses an apparatus for forming a substantially continuous filament of a thermoplastic work material and for imparting a swirling motion to the filament. The configuration of a material discharging nozzle and the construction of the system for supplying a compressed gas are selected in such a way that the filament of thermoplastic material is imparted a swirling motion so that it adheres to a surface in the form of a series of arcs. This is achieved by causing compressed air to flow in a bore extending in parallelism with the filament of thermoplastic material and discharging into a ring-shaped space which is in direct communication with the outlet for air. Swirling motion can be imparted in a number of ways, for example, by providing a gas- or material-discharging bore which is inclined with reference to the longitudinal axis of the apparatus. Alternatively, the material discharging orifice is asymmetrical with reference to the outlet for air.
The swirling motion which is imparted to the filament of thermoplastic material causes such material to deposit on a selected surface in the form of arcs having identical diameters. Thus, if such arcuate deposits of thermoplastic material are to form a continuous strip, they must closely overlap with the result that the marginal zones of the developing strip are higher (i.e., they contain more material) than the median zone. Consequently, if the strip is brought into contact with another surface, the bond between the surface to which the thermoplastic material was applied and the other surface is established primarily along the marginal portions of the strip. This is undesirable irrespective of whether the thermoplastic material is an adhesive or a sealing compound. Moreover, the quantity of thermoplastic material which can be applied per unit of time is relatively small which is unsatisfactory in many, or most, instances if the thermoplastic material is a sealing compound. If the quantity of applied thermoplastic material is raised above a relatively low upper limit, the applied material is likely to be sprayed beyond the selected area and to thus contaminate the workpiece.